1. Field of the Invention
The present invention relates to an electrostatic image developing toner for use in electrophotographic image formation.
2. Description of the Related Art
User requirements concerning electrophotographic apparatuses (electrophotographic image-forming apparatuses), such as copying machines and printers, are becoming increasingly complex year by year. There is a recent demand for high durability to withstand high-speed printing and consistent image quality independent of the operating environment (temperature and humidity).
It is known that image quality can be improved by the addition of a submicron external additive to toner base particles. Japanese Patent Laid-Open No. 2007-171666 discloses a technique of adding submicron silica particles to toner base particles to decrease the physical adhesion strength between toner and a transfer member, thereby improving toner transferability and image quality.
However, submicron silica particles tend to separate from toner particles because of stress in a developing apparatus. The separation of silica particles from toner particles often results in low toner transferability. Silica particles separated from toner particles may cause soiling of a member, such as a charge member, during printing of many copies. Soiling of such a member tends to reduce image quality and makes it difficult to maintain high image quality in long-term image output.
It is known that such a problem can be solved by the internal addition of silica particles to toner base particles. Japanese Patent Laid-Open No. 2006-220673 discloses a technique of internally adding silica particles to toner base particles to fix the silica particles on a surface layer of each of the toner base particles in the production of toner particles in an aqueous medium, thereby preventing the silica particles from separating from the toner base particles.
In the production of toner particles in an aqueous medium, however, in order to internally add silica particles to toner base particles and place the silica particles on the outermost surface layer of each of the toner base particles, it is necessary to reduce the degree to which the silica particles are subjected to hydrophobic treatment so as to increase the hydrophilicity of the silica particles.
Reduction of the degree to which the silica particles are subjected to hydrophobic treatment, however, tends to increase the hygroscopicity of the silica particles. This often lowers toner chargeability in a high temperature and high humidity environment, possibly causing fogging due to charging failure.
Thus, a method of internally adding silica particles to toner base particles to fix the silica particles on a surface layer of each of the toner base particles, thereby preventing the silica particles from separating from the toner base particles is effective in the production of toner particles in an aqueous medium.
However, there is still room for improvement in order to maintain high toner transferability and suppress poor image quality, such as fogging, even in long-term image output while the decrease in toner chargeability is suppressed even in a high temperature and high humidity environment.